Get yourself a strong vacuum cleaner with a plastic nozzle that will go over the lens assembly when it is extended, turn the vac on and lightly tap the side of the camera while zooming in and out. This worked well on my P100 that had dirt in the lens.

Thank you so much for the vacuum cleaner suggestion! I had tried blasting air into my P150 and even tried removing the screws and opening the shell (this still doesn't let you get at the sensor or lens, and I didn't have the nerve to take it apart further).

The vacuum cleaner succeeded in removing the spot of dust which had been plaguing my pictures for 2 weeks. I had even started shopping for a new camera but am now much happier keeping my P150!
-Ralph